The present invention relates to preparation of patterned reticles to be used as masks in the production of semiconductor and other devices. Methods and devices are described utilizing resist and transfer layers over a masking layer on a reticle.
Semiconductor devices include multiple layers of structures. The structures are formed in numerous steps, including steps of applying resist, then exposing, developing and selectively removing the resist to form a pattern of exposed areas. The exposed areas may be etched to remove material or sputtered to add material. A critical part of forming the pattern in the resist is exposing it. Resist is exposed to an energy beam that changes its chemical properties. One cost-effective way of exposing the resist is with a stepper. A stepper uses a reticle, which typically includes a carefully prepared, transmissive quartz substrate overlaid by a non-transmissive or masking layer that is patterned with areas to be exposed and areas to be left unexposed. Patterning is an essential step in the preparation of reticles. Reticles are used to manufacture semiconductor and other devices, such as flat-panel displays and television or monitor screens.
Semiconductor devices have become progressively smaller. The feature dimensions in semiconductor devices have shrunken by approximately 40 percent every three years for more than 30 years. Further shrinkage is anticipated. Current minimum line widths of approximately 0.13 microns will shrink to 0.025 microns, if the historical rate of development continues for another 15 years.
The pattern on a reticle used to produce semiconductor devices is typically four times larger than that on the wafer being exposed. Historically, this reduction factor has meant that minimum feature dimensions in the reticles are less critical than the minimum feature dimensions on the surface of the semiconductor. However, the difference in criticality is much less than might be expected and will in the near future disappear.
Critical dimension uniformity, as a percentage of line width, is more exacting in the pattern on a reticle than in the features on the surface of a wafer. On the wafer, critical dimension uniformity of plus or minus 10 percent of the line width has historically been acceptable. In the error budget for the wafer line width, the mask has been allowed to contribute half of the critical dimension variation, or a variation of five percent of a line width. Other factors use the remaining error budget. It has been observed that nonlinearities in transfer of a pattern from a reticle to a wafer magnify any size errors in the mask. This is empirically quantified as a mask error enhancement factor (MEEF or MEF). In current technology, the mask error enhancement factor is typically two. Therefore, the critical dimension uniformity on the reticle is reduced to approximately two and one-half percent of a line width, to remain within the error budget.
It is anticipated that requirements for critical dimension uniformity will tighten in time, particularly for masks. On the surface of the wafer, a critical dimension uniformity of plus or minus five percent of the line width will be required in the future. At the same time, the mask error enhancement factor is likely to increase due to more aggressive lithographic process trade-offs, such as tuning the lithographic process to optimize the manufacture of contact holes, transistors or other critical features in order to use feature sizes closer to the theoretical resolution limit. For masks, a critical dimension uniformity of plus or minus one percent of a line width or feature size is anticipated. At this rate, the tolerance for critical dimension errors on the mask will be smaller in absolute nanometers than it is on the surface the wafer, despite the fact that the stepper takes advantage of a mask that is four times as large as the area on the wafer that is being exposed.
One of the energy beam sources currently used to expose resist is deep ultraviolet (DUV), in the wavelength range of 100 to 300 nanometers. This energy source is used with two types of resist to produce masks: conventional positive, so called Novolac-DNQ, resist and chemically amplified resist. Essentially all DUV exposure in steppers uses chemically amplified resist. The requirements in pattern generators for patterning of reticles are so different than in steppers that chemically amplified resists are unsuitable for patterning reticles. Work to modify conventional Novolac-DNQ resist to produce a resist suitable for DUV exposure of mask patterns reportedly has failed.
Uniformity and feature size requirements have become so demanding that wet etching no longer is suitable. Wet etching is generally not useable when the size of features approach the thickness of the films the features are etched from. A wet etch etches sideways as much as it etches vertically. Deterioration of the three-dimensional shape of small features results. When chrome is wet etched with resist as an etch mask, the etchant removes chrome under the resist, referred to as undercutting. Clear areas produced by wet etching chrome with a resist mask typically come out 0.2 microns too large. A wet etched resist image with alternating lines and spaces equally 0.4 microns wide, produces a chrome mask pattern where the spaces (clear) are 0.6 microns wide and the lines (dark) are 0.2 microns. This is a large deviation. It is difficult to compensate for this deviation by changing the data or the dose. For smaller features, narrow lines will simply disappear. Therefore any pattern with features smaller than 0.5-0.6 microns wide needs to be produced by dry or plasma etching. The plasma process used to etch chrome produces vertical xe2x80x9cline-of-sightxe2x80x9d etching characteristics. The chrome is removed only where it is within the line of sight from the plasma source; essentially no undercutting results.
Issues Using Positive Non-Amplified Resists
Positive non-amplified resists provide excellent performance in the violet visible and near UV wavelength ranges. This resist is transparent and has high contrast, giving essentially vertical resist walls and good process latitude. It has good shelf life and mask blanks can be precoated with resist at the time of manufacturing, shipped to users, and kept in storage until needed. Although there is a small decay of the latent image, plates can in principle be exposed today and developed after weeks.
In the DUV wavelength range, both the Novolac resin and the photoactive compound used in Novolac absorb strongly. The edge wall angle after development is partly controlled by the absorption of light and partly by the resist contrast. With high absorption, the features will have strongly sloping edge walls, whatever the chemical contrast. No non-amplified resist formulation is known which combines good contrast with high transparency.
The effect of non-vertical trench walls is significant for narrow lines. One reason for non-vertical trench walls is that a resist layer is eroded by the plasma during the etching. The uniformity of resist erosion is difficult to control since, among other things, it depends on the pattern to be etched. Erosion makes the clear areas larger and varying plasma activity from run to run and across the surface of the workpiece gives a varying CD between masks and within each mask. The variation of the resist thickness at the end of the plasma etching step may be 50 nm peak-to-valley or more. For a wall angle of 80 degrees, instead of 90 degrees, a 50 nm variation in resist thickness produces a variation in trench width, at the bottom of the trench, of nearly 20 nm, which may translate into an undesirable three-sigma deviation of 20 nm. This erosion problem is exacerbated by the high optical absorption of non-chemically amplified resists used with DUV radiation. High optical absorption leads to greater development of the resist at the top of the trench than the bottom, further increasing the variation in line width.
Resist sidewall deviation from 90xc2x0 vertical inevitably limits the line resolution. In 0.5 micron thick resist layer with a side wall angle of 80 degrees, a line having a width of 0.025 microns at the top of the resist layer is only 0.2 microns wide at the bottom of the resist layer where the chrome is etched. Using current chemistry it is not possible to make the resist thinner than 0.4-0.5 microns and still protect the chrome during the dry etching. If the line at the top of the resist layer is narrowed, the wall angle less favorable or the resist thicker, the line would tend to vanish.
Obviously, each of the problems described gets worse as line widths get smaller, tolerances diminish, and the wavelength move into the deep ultraviolet.
Issues With Chemically Amplified Resists
The use of chemically amplified resists introduces other problems. Chemically amplified resists developed for stepper processing are transparent and have high contrast, giving almost perfectly vertical resist walls. However, they need a thermal annealing or activation step after exposure, that is, a post-exposure bake (PEB). Activation and chemical amplification are highly sensitive to the temperature in time of this bake. Use of chemically amplified resists on reticles is much more difficult than on wafers, due to the thickness and shape of reticles. Reticles are much thicker and less thermally conductive than silicon wafers, making it more difficult to control the baking sequence accurately. Furthermore, reticles are square, leading to corner effects that are not experienced with round wafers. These post-exposure bake problems are not necessarily limited to chemically amplified resists, but are particularly had for chemically amplified resists due to the criticality of the baking step. Non-chemically amplified resists are sometimes baked after exposure to even out standing wave interference effects, leading to the same problems. Post-exposure baking also introduces a latent image diffusion problem, for both kinds of resist, but worse for the chemically amplified resists since the deactivation post bake often requires a substantially different temperature than that optimized for standing wave reduction.
An additional problem of chemically amplified resists is their instability and short working life. Chemically amplified resists have been developed for use with steppers, which can finish 100-500 wafers in the same time that a mask writer produces one mask. Chemically amplified resists are spun on the surface of wafers and prebaked shortly before they are placed in the stepper and are baked shortly thereafter on an automated line, within the relatively short working life of the resist. This makes the current generation of chemically amplified resists unsuitable for use in a mask writer, which may take one to ten hours or more to write a mask and typically operates without an automated processing line. The related problem is that the time from prebaked to post bake depends on the pattern written and is highly variable. As more suitable chemically amplified resists are developed for use with mask making, it will be necessary to take into account the substantial variation in mask writing the time.
Issues Common To All Single-Layer Resists
All single layer resists share properties that makes them less suitable in the foreseeable future. The mask pattern is always wet developed since there exists no process for dry development. Again, the minimum resist layer thickness essentially constant at 0.4-0.5 microns, regardless of the feature size, in order to resist plasma erosion in areas where chrome is not supposed to be removed. As features get very small, wet developed resist structures assume an unfavorable aspect ratio. In a mask pattern with 10 billion features it is highly likely that some of these high aspect ration features will be damaged by hydrodynamic forces and surface tension during wet processing.
For optical exposure, single layer resists further require a trade-off between transparency and interference effects. The thin resist layer needs to be transparent to be exposed from top to bottom, but the transparency makes it subject to optical interference that lowers the effective performance of the resist and increases process variability. Two interference effects are sometimes referred to as standing wave and bulk effects. The standing wave effect results from interference within the resist layer between light directed toward the reticle""s surface and light reflected back. The light directed toward the non-transmissive, mirror-like masking layer and the light reflected back from that layer produce a standing wave where the crests and troughs of the directed and reflected light align. This produces vertical bands of more and less completely exposed resist. When the resist is developed and selectively removed, there is a tendency for the sides of the resulting trench to bend in and out, which is referred to as the standing wave effect. The related bulk effect results from interference above the resist layer between light reflected off the surface of the resist and light reflected off the surface of the reticle and back out of the resist. With certain thicknesses of resist, there is destructive interference between the light entering and leaving the resist, allowing a maximum number of photons to stay in the resist layer, producing high sensitivity. Variations in the resist bulk or thickness effect the sensitivity of the film and lead to non uniformity is in the pattern produced. As a resist film is made more transparent, interference effects are reduced, but the etch slope would be worse. These problems are common to ordinary and chemically amplified resists. In wafer lithography the dilemma is normally solved with a thin anti reflecting coating under the resist and sometimes also on top of the resist as well.
Mask production faces additional issues. For instance, production control is difficult due to low production volume. Monitoring and feedback techniques used to improve the quality of semiconductor production are not readily applied to low volume production. Thus, a mask shop needs a more stable process through a semiconductor fab.
Thus, it is desirable to develop a new process for patterning reticles and forming phase shift windows in reticles. The new process preferably would be suitable for non-chemically amplified resists or yet to be developed amplified resists and would yield very small feature sizes with great uniformity by avoiding interference effects and other process hazards.
An objective of the invention is to produce small features on a reticle with precise critical dimensions, using a technique suitable to a variety of energy sources.
One embodiment of the present invention includes a method of creating a patterned reticle, including creating a latent image in a resist layer using a pattern generator, creating a plasma etch barrier corresponding to said latent image, directionally etching the transfer layer through said plasma etch barrier, and removing the transfer layer to expose unetched portions of the masking layer. According to this embodiment, the resist layer maybe wet developed. It may be less than 200 nm thick and preferably 150 nm thick. The transfer layer maybe between 200 and 500 nm thick, and preferably 350 nm thick. The plasma etch barrier may comprise silicon in the resist layer, which may be present before the latent image is created or may be added after it is created. Alternatively, the plasma etch barrier a comprise a separate film between the resist and transfer layers, preferably deposited by sputtering. This etch barrier film may be a metal containing film, comprising aluminum, a metal oxide, silicon, or silicon oxide. A plasma etch barrier comprising a separate film may be patterned by plasma etching through the resist layer. A further aspect of this embodiment is that the transfer layer maybe essentially non-transmissive to an energy beam used to create the latent image. This transfer layer may be removed using a first plasma chemistry. The first plasma chemistry may contain halogen ions and maybe an oxygen plasma. The transfer layer maybe an organic material. Directional etching of the transfer and masking layer may be carried out by RIE type etching and the transfer layer may be removed by non-preferential oxygen plasma.
Additional embodiment of the present invention includes creating features on a mask blank, including the steps of exposing a resist layer using a pattern generator, developing the resist layer and selectively removing portions thereof, directionally etching a transfer layer underneath the resist layer, directionally etching a masking layer underneath the transfer layer, and removing the transfer layer to expose unhedged portions of the masking layer. The pattern generator uses may use photon energy, electron beams, or particle beams. When photon energy is used, the transfer layer maybe essentially non-transmissive to the wavelength of photon energy used. A variety of wavelengths can be used to create a variety of minimum feature dimensions, because there is a critical relationship between wavelength and resulting feature size. Energy of 300 to 380 nm wavelength can be used to create minimum feature dimensions and 75 to 285 nm. Energy of 200 to 300 nm can be used to create minimum feature dimensions of 55 to 225 nm. Energy of 100 to 220 nm can be used to create minimum feature dimensions of 32 to 124 nm. Energy of five to the 13 nm can be used to create minimum feature dimensions of 6244 nm. When the electron beam is used, less than 3000 eV energy is preferred. Minimum feature dimensions of 2270 nm to be created. Depending on type of energy beam used, the minimum feature dimensions created may be in the ranges of 75-285 nm, 55-225 nm, 32-124 nm, or 6-44 nm. An aspect of this embodiment is that the pattern generator can be aligned to the reticle when the resist and transfer layers are transmissive to a certain, non-exposing wavelength of light by observing features beneath the resist and transfer layers. With a transfer layer that is more absorptive than the resist layer to another certain wavelength of light, the pattern generator can autofocus on the interface between the resist and transfer layers.
According to a further aspect of the invention, multiple passes may be used to expose the resist layer, preferably four passes. The exposure passes showed take place in essentially opposing directions, yielding an average time between exposure and completion of the final exposure passes which is essentially equal for locations dispersed across the reticle.
Additional aspect of the present invention is that a plasma comprising oxygen and silicon dioxide can be used to selectively remove a silicon containing resist. The resist can be treated with silicon prior to developing. Useful silicon treating compounds includes silane, liquid compounds and gaseous compounds. The silicon can be treated after development and before removal of the resist. Resist development can be carried out by wet or dry development.
Either embodiment of the present invention can be enhanced by a including in steps of inspecting and repairing the selectively removed resist. Alternatively, the developed resist can be inspected and features precisely widened to match a critical tolerance.
The directional etching of the transfer or masking layers according to either embodiment can be carried out by plasma etching or reactive ion etching. Chlorine may be used in etching gas to remove a masking layer. The transfer layer may comprise an organic material, preferably one adapted to planarizing the masking layer and dyed with a DUV-absorbing dye. The masking layer may comprise more than one physical layer, for instance a layer of chrome overlaid by an antireflective layer of nonstoichiometric chrome oxide.
Another embodiment of present invention is a method of preparing a reticle blank for patterning, including the steps of forming a masking layer a reticle substrate, spinning an organic layer over the masking the layer, baking the organic layer, spinning a positive silicon-containing resist layer over the organic layer, and baking the resist layer. According to this embodiment, the masking layer may be comprised of chrome in the range of 40-90 nm thick. Alternatively, it may comprise aluminum or tungsten. On a quartz reticle substrate, the masking layer may comprise a patterned structure. The resist layer maybe between 50 and 200 nm thick, preferably 150 nm thick. The resist and transfer layers may have different characteristics of absorbing certain wavelengths of light, so that a pattern generator can focus on the interface between these layers.